The present invention relates to the formation of soot used in the manufacture of glass and, more particularly, to a method and apparatus for the delivery of liquid precursors to a flame during flame hydrolysis.
While the invention is subject to a wide range of glass soot deposition applications, it is especially suited for making soot for use in the manufacture of optical waveguides, and will be particularly described in that connection.
Various processes are known in the art that involve the production of metal oxides from vaporous reactants. Such processes require a feedstock solution or precursor, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site (also known as a soot reaction zone to those skilled in the art), and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected on any deposition receptor in any number of ways ranging from a collection chamber to a rotating mandrel. The collected soot may be simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. This process is usually carried out with specialized equipment having a unique arrangement of nozzles and burners.
Much of the initial research that led to the development of such processes focused on the production of bulk silica. Selection of the appropriate feedstock was an important aspect of that work. Consequently, it was at that time determined that a material capable of generating a vapor pressure of between 200-300 millimeters of mercury (mm Hg) at temperatures below approximately 100xc2x0 C. would be useful for making such bulk silica. The high vapor pressure of silicon tetrachloride (SiCl4) suggested its usefulness as a convenient vapor source for soot generation and launched the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other is responsible for the presently accepted use of SiCl4, GeCl4, POCl3, and BCl3 as feedstock vapor sources.
Use of these and other halide-based feedstocks as vapor sources, however, does have its drawbacks. The predominate drawback being the formation of hydrochloric acid (HCl) as a by-product of oxidation. HCl is not only detrimental to the deposition substrates and the reaction equipment, but to the environment as well. Overcoming this drawback, amongst others, led to the use of halide-free compounds as precursors or feedstocks for the production of soot for optical waveguides.
Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, avoids the formation of HCl, other problems remain, particularly when the glass is intended for the formation of optical waveguides and high purity silica soot. It has been found that, in the course of delivering a vaporized polyalkylsiloxane feedstock to the burner, high molecular weight species can be deposited as gels in the line carrying the vaporous reactants to the burner, or within the burner itself. This leads to a reduction in the deposition rate of the soot that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. It also leads to imperfections in the blank that often produce defective and/or unusable optical waveguide fiber from the affected portions of the blank. An additional problem encountered while forming silica soot using siloxane feedstocks is the deposition of particulates having high molecular weights and high boiling points on the optical waveguide fiber blank. The build-up of these particulates results in xe2x80x9cdefectxe2x80x9d or xe2x80x9cclustered defectxe2x80x9d imperfections that adversely affect the optical and structural quality of optical waveguides formed using the silica soot.
Defects typically are in the form of small (i.e. 0.1 to 4.0 mm in diameter) bubbles in a glass body. They are often formed in fused silica by an impurity, such as uncombusted gelled polyalkylsiloxane. A very small particle of siloxane gel can be the initiation site for such a defect. Since siloxane decomposes at high temperature after being deposited on the glass body; it can give off gases that cause the formation of the defect. Clustered defects are larger glass defects found in optical waveguide fiber preforms, and often occur as a series of defects in the form of a line or a funnel-or flower-shaped cluster. Typically, a large particle of gel is the initiation site for a clustered defect. After the gel particle has struck the porous preform, it causes a raised area to stand out on the preform surface. Because the clustered defect is a raised site, more heat transfer passes to this site. Due to this increased heat transfer, more thermophoresis occurs at the site, causing the imperfection to grow and leave behind a string of defects. As a result of the clustered defect, the affected portion of the optical waveguide preform cannot be consolidated normally, and the consequent irregularity in the blank yields a defective optical waveguide. For example, in the case of a typical 100 kilometer consolidated waveguide fiber blank, which has a diameter of 70 millimeters (mm) and a length of 0.8 meter (m), the presence of one clustered defect on the surface of the blank will typically result in the loss of 5 kilometers of optical waveguide fiber during drawing. In the case of a larger consolidated blank, the negative impact of a single clustered defect is proportionately higher. In a 250 kilometer consolidated blank, which has a diameter of 90 mm and a length of 1.8 m, one clustered defect on the surface of the blank will typically result in the loss of 8 kilometers of optical waveguide fiber during drawing.
U.S. patent application Ser. No. 08/767,653, discloses that clustered defects can be reduced by delivering a liquid siloxane feedstock to a conversion site, atomizing the feedstock at the conversion site, and converting the atomized feedstock at the conversion site into silica. Because the precursors are delivered directly into a burner flame as a liquid rather than a vapor, the vapor pressure of the precursors is no longer a limiting factor in the formation of soot for optical waveguides. However, the external atomizers and their methods of use disclosed in application Ser. No. 08/767,653 are not without limitation. External atomizers typically have a liquid discharge orifice that is co-planer or substantially co-planar with the burner face. Accordingly, the liquid and the atomizing gases come together at the surface of the burner. Since the flame is generated adjacent the face of the burner, atomization must occur very quickly if the liquid is to be dispersed into droplets prior to reaching the flame. For this to occur, very high atomizing gas velocities are required. While these high gas velocities can disperse the liquid into small droplets, they do so by creating turbulence, which in turn adversely affects the soot deposition rate.
Additionally, external atomizers rely on a very small atomizing gas annulus positioned around the liquid exit orifice to provide the high velocity atomization gas that impinges on the liquid. As a result, the close proximity of the liquid exit orifice and the soot reaction zone render both the liquid exit orifice and the atomizing gas annulus of the external atomizers susceptible to soot build up and clogging. When either the annulus or the liquid exit orifice is partially clogged by this soot build up, the flame, and thus the soot stream, becomes non-uniform and the soot deposition rate suffers. Because of the small size of these openings, cleaning of the external atomizer is both time consuming and difficult. Moreover, because the burners are shut down during cleaning operations, production down time has a significant adverse economic impact on operations.
The external atomizer is also expensive to manufacture and limited in flexibility. Since the liquid exit orifice on the burner face of the external atomizer is generally provided with a knife-edge to facilitate rapid mixing of the atomizing gases and the liquid stream discharged from the orifice, the liquid exit orifice diameter is limited to a dimension which is greater in size than preferred. Additionally, the liquid exit orifice must be centrally positioned within the annulus to avoid the problem of non-concentricity. Any slight misalignment, and the flame will be non-concentric. Non-concentricity results in poor soot deposition and is a serious problem during laydown. Accordingly, tolerances must be tight, which in turn increases manufacturing costs.
There is a need therefore for alternative methods and apparatus for depositing glass precursor soot, especially for optical fiber and other waveguide related applications.
The present invention is directed to an improved method and apparatus for delivering a liquid precursor to a burner flame to form soot used in the manufacture of glass. The liquid precursor, capable of being converted by thermal oxidative decomposition to glass, is provided and introduced directly into the flame of a combustion burner, thereby forming finely divided amorphous soot. The amorphous soot is typically deposited on a receptor surface where, either substantially simultaneously with or subsequent to its deposition, the soot is consolidated into a body of fused glass. The body of glass may then be either used to make products directly from the fused body, or the fused body may be further treated, e.g., by forming an optical waveguide such as by drawing to make optical waveguide fiber as further described in U.S. patent application Ser. No. 08/574,961 entitled, xe2x80x9cMethod for Purifying Polyalkylsiloxanes and the Resulting Productsxe2x80x9d, the specification of which is hereby incorporated by reference.
The burner assembly for delivering the precursor directly to the burner flame as an atomized liquid includes a novel recessed injector orifice that delivers a precision stream of liquid into a chamber where the stream is exposed to a low velocity atomizing gas. The introduction of the gas and liquid stream into the chamber increases the pressure within the chamber causing the liquid stream to be discharged as an aerosol from an exit orifice at the burner face of the atomizing burner assembly, which, in operation, generates a conversion site flame. The small liquid droplets of the aerosol are thus fed into the flame to convert the compound by thermal oxidative decomposition to finely divided amorphous soot. Generally, a receptor surface such as a rotating mandrel is positioned in close proximity to the atomizing burner assembly to permit deposition of the soot on the receptor surface.
The principal advantage of the present invention is the provision of an arrangement and method which substantially reduces the clogging of the orifices on the face of the burner assembly by using lower atomizing gas velocities than other atomizing burner assemblies known in the art, while at the same time, atomizing an equivalent liquid flow rate. By using lower atomizing gas velocities, turbulence is reduced at the soot reaction zone, and thus the soot deposition rate is greatly improved. Moreover, by delivering the precursor to the flame as a liquid rather than as a vapor, gelling of the precursor is prevented in that exposure of the precursor to the high temperature environments of a vaporizer and vapor delivery system are avoided. This improves the yield and quality of the soot produced and also reduces the maintenance requirements of the production system. Additionally, because the precursor does not have to reach the vapor phase prior to its exposure to the burner flame, elements never before used to form soot for use in the manufacture of the preforms can now be converted by oxidation or flame hydrolysis to soot for use in the manufacture of glass preforms, including elements selected from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, VA, and the rare earth series of the Periodic Table of Elements.
In another aspect, the invention includes a burner assembly formed from a housing having a burner face with multiple gas orifices and an atomization orifice. An injector chamber resides within the housing and a plurality of gas passageways within the housing communicate with the gas orifices. An injector having an injector orifice is positioned within the injector chamber such that the injector orifice is remotely spaced from the atomization orifice. The injector, together with the housing, forms a pressurization chamber within the burner assembly.
In yet another aspect of the present invention, the burner assembly includes and injector constructed and arranged to deliver liquid precursors, and a housing substantially surrounding the injector. The housing has a burner face, which includes an orifice rim defining an atomization orifice. The orifice rim is shaped such that turbulence is reduced as the liquid precursor is discharged from the atomization orifice.